1. Field of the Invention
The present invention relates to a novel silicon compound, a method for making the same, and an electroluminescent device using the same.
2. Description of the Related Art
High performance electronic devices require materials for effectively transporting only electrons or holes. Hole-transporting materials which transport only holes at a high efficiency have been widely used for electrophotographic photosensitive members and are expected as organic luminescent materials. For example, use of organic photoconductive (OPC) materials is rapidly spreading as photosensitive materials in electrophotography and the like. OPC materials have advantages in safety and productivity and costs compared to inorganic photosensitive materials, and have been vigorously used for copying machines and printers.
The OPC members generally have a multilayered structure of a charge generation layer (CGL) and a charge transport layer (CTL). The CTL is formed by coating a dispersion of a low molecule charge transport material (CTM), e.g. a triarylamine, hydrazone or dialkylaminobenzene derivative, in a transparent polymeric film, such as a polycarbonate resin. The concentration of the CTM in the polymer matrix is limited since the CTM crystal deposits at a high concentration. On the other hand, hole transferability decreases as the concentration decreases. Further, the CTL is fragile and has a low tensile strength. Such low mechanical strength causes scratches and cracks on the OPC member, resulting in image defects.
Exemplary polymeric charge transport materials are polystyrenes having hydrazone groups which are disclosed in Japanese Patent Laid-Open Nos. 2-272005 and 3-180852. These materials are formed into a film with great difficulty, and have unsatisfactory hole transport rates and residual potentials.
Pope et al., first discovered an electroluminescence (EL) of an organic material, that is, single-crystal anthracene in 1963 (J. Chem. Phys., 38, 2042 (1963)). Helfinch and Schneider succeeded observation of relatively strong EL in an injection EL material containing a solution electrode system having a high injection efficiency in 1965 (Phys. Rev. Lett., 14, 229 (1965)). Many studies of organic luminescent materials containing conjugated organic hosts and conjugated organic activators having fused benzene rings have been disclosed in U.S. Pat. Nos. 3,172,862, 3,173,050, and 3,710,167; J. Chem. Phys., 44, 2902 (1966); J. Chem. Phys., 58, 1542 (1973); and Chem. Phys. Lett., 36, 345 (1975). Examples of disclosed organic hosts include naphthalene, anthracene, phenanthrene, tetracene, pyrene, benzpyrene, chrysene, picene, carbazole, fluorene, biphenyl, terphenyl, triphenylene oxide, dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene. Examples of disclosed activators include anthracene, tetracene and pentacene. Since these organic luminescent materials are provided as single layers having a thickness of more than 1 xcexcm, a high electric field is required for luminescence. Under such a circumference, thin film devices formed by a vacuum deposition process have been proposed (for example, xe2x80x9cThin Solid Filmsxe2x80x9d p. 171 (1982); Polymer, 24, 748 (1983); and J. Appl. Phys., 25, L773 (1986)). Although the thin film devices are effective for reducing the driving voltage, their luminance is far from a level for practical use.
Tang et al. developed an EL device having a high luminance for a low driving voltage (Appl. Phys. Lett., 51, 913 (1987) and U.S. Pat. No. 4,356,429). The EL device is fabricated by depositing two significantly thin layers, that is, a charge transport layer and a luminescent layer, between the positive electrode and the negative electrode by a vacuum deposition process. Such layered organic EL devices are disclosed in, for example, Japanese Patent Laid-Open Nos. 59-194393, 3-264692, and 3-163188, U.S. Pat. Nos. 4,539,507 and 4,720,432, and Appl. Phys. Lett., 55, 1467 (1989).
Also, an EL device of a triple-layered structure having independently a carrier transport function and a luminescent function was disclosed in Jpn. J. Apply. Phys., 27, L269 and L713 (1988). Since the carrier transportability is improved in such an EL device, the versatility of possible dyes in the luminescent layer is considerably increased. Further, the device configuration suggests feasibility of improved luminescence by effectively trapping holes and electrons (or excimers) in the central luminescent layer.
Monolithic organic EL devices are generally formed by vacuum deposition processes. EL devices having considerable luminance are also formed by casting processes (as described in, for example, Extended Abstracts (The 50th Autumn Meeting (1989), p. 1006 and The 51st Autumn Meeting (1990), p. 1041; The Japan Society of Applied Physics). Considerably high luminance is also achieved by a single-layered mixture-type EL device, in which the layer is formed by immersion-coating a solution containing polyvinyl carbazole as a hole transport compound, an oxadiazole derivative as a charge transport compound and coumarin-6 as a luminescent material (as described in Extended Abstracts (The 38th Spring Meeting (1991), p. 1086; The Japan Society of Applied Physics and Related Societies).
As described above, the organic EL devices have been significantly improved and have suggested feasibility of a wide variety of applications; however, these EL devices have some problems for practical use, for example, insufficient luminance, a change in luminance during use for a long period, and deterioration by atmospheric gas containing oxygen and humidity. Accordingly, a novel material not having such disadvantages and an electroluminescent device have been eagerly awaited.
It is an object of the present invention to provide a silicon compound having excellent charge transport characteristics and durability.
It is another object of the present invention to provide a silicon compound capable of easily forming a uniform film.
It is a further object of the present invention to provide an electroluminescent device emitting light with high luminance and a high efficiency and having high durability.
It is a still further object of the present invention to provide an electroluminescent device emitting light with a wide variety of wavelengths and hues.
It is another object of the present invention to provide an electroluminescent device easily produced at relatively low production costs and having a high degree of safety.
An aspect of the present invention is a silicon compound having a repeating unit represented by the following general formula (1): 
wherein R is a hydrogen atom, a straight or branched alkyl group having 1 to 20 carbon atoms, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group, Ar is a substituted or unsubstituted aryl group or an unsubstituted heterocyclic group, m is an integer of 2 or more, and n is an integer of 5,000 or less.
Another aspect of the present invention is a method for synthesizing of a silicon compound having a repeating unit represented by the following general formula (1) by reacting a compound represented by the general formula (2) with a compound represented by the general formula (3): 
wherein R, Ar, m, and n are the same as above; 
wherein R is a hydrogen atom, a straight or branched alkyl group having 1 to 20 carbon atoms, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group, and n is an integer of 5,000 or less;
CH2xe2x95x90CHxe2x80x94(CH2)mxe2x88x922xe2x80x94Arxe2x80x83xe2x80x83(3)
wherein Ar is a substituted or unsubstituted aryl group or an unsubstituted heterocyclic group, and m is an integer of 2 or more.
A further aspect of the present invention is an electroluminescent device comprising a pair of electrodes and an organic compound layer interposed between said electrodes, said organic compound layer comprising a silicon compound having a repeating unit represented by the following general formula (1): 
wherein R is a hydrogen atom, a straight or branched alkyl group having 1 to 20 carbon atoms, a substituted or unsubstituted aryl group, or a substituted or unsubstituted heterocyclic group, Ar is a substituted or unsubstituted aryl group or an unsubstituted heterocyclic group, m is an integer of 2 or more, and n is an integer of 5,000 or less.
The silicon compound in accordance with the present invention has excellent charge transport characteristics and can form a uniform and smooth film, hence it is useful as a charge transport material.
The method in accordance with the present invention enables ready production of the silicon compound.
The electroluminescent device in accordance with the present invention has high luminance at a low voltage for long periods, and can emit a variety of hues. For example, the electroluminescent device can emit primaries, i.e., red, blue and green; hence it can be used in displays. The device can be produced by a vacuum deposition or casting process at low production costs.